The Gastrointestinal (GI) Syndrome is a primary complication of cancer treatments, associated with delivery of therapeutic doses of chemotherapy and radiation, and also constitutes a potentially lethal result of accidental ionizing radiation (IR) exposure. The syndrome results from depletion of the stem cell compartment of the GI tract, resulting in denudation of the GI epithelium, and death due to lack of nutrient adsorption, diarrhea and dehydration, electrolyte imbalance and enterobacterial sepsis. There is no effective approach to mitigate the basic underlying pathology of the GI Syndrome. The Project BioShield Act, part of a strategy to defend American soil from threat of a limited nuclear attack or nuclear accident, emphasizes the need for research towards development of effective countermeasures to mitigate GI tract lethality in the general population at least 24 h after radiation exposure. Evidence that microvascular dysfunction within the crypt-villus unit regulates GI tract lethality identified a novel target for mitigation of the GI Syndrome. This vascular dysfunction, mediated by generation of the pro-apoptotic sphingolipid ceramide on the outer leaflet of the endothelial plasma membrane and subsequent ceramide-driven membrane reorganization into ceramide-rich platforms (CRPs), is amenable to pharmacologic inactivation. Based upon these discoveries, mouse 2A2 (m2A2) anti- ceramide IgM antibody was developed to bind and neutralize ceramide, and inhibit IR-induced tissue damage in vivo. m2A2 significantly improved GI tract stem cell survival and promoted tissue recovery, both when administered prior to or up to 24 hours following lethal radiation exposure. Critically, 2A2 mitigated both sublethal and lethal GI tract damage. These data indicate that anti-ceramide Abs represent a novel class of pharmacologic agents for mitigation of the GI Syndrome, and for use as radiological countermeasures. Humanized 2A2 has since been generated, and confirmed to be an effective mitigator of crypt lethality. In the current application, we propose to identify the most promising radiation mitigating therapeutic from a panel of humanized anti-ceramide reagents, including anti-ceramide Abs and antibody fragments, for advancement into preclinical development. PUBLIC HEALTH RELEVANCE: The Department of Health and Human Services has emphasized the need for radiation biology research towards development of effective countermeasures to mitigate gastrointestinal (GI) tract lethality in the general population at least 24 h after radiation exposure. Recent evidence demonstrates that neutralization of the membrane lipid ceramide with a monoclonal antibody specific for ceramide protects GI stem cells from death and promotes organ regeneration following high-dose radiation exposure, even when administered 24 hours following exposure. This research proposal aims to identify the most effective radiation mitigator from a panel of humanized anti-ceramide reagents for advancement into preclinical development.